This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In a variety of fluid handing systems, the flow of a fluid through a pipe is controlled by a gate valve. These valves typically include a moveable gate, static seat rings that seal against the gate, and a housing in which the gate and seat rings are disposed. Generally, the gate includes two-opposing faces that each contact a seat ring and a flow bore that extends between the faces. To conduct flow through the valve, the flow bore is slid into alignment with the seat rings, and to restrict the flow, it is slid out of alignment with the seat rings. As the gate moves from the sealed position to the unsealed position, it slides along a generally straight line between the seat rings, which are typically affixed to the housing.
In some applications, including applications in which metal-to-metal sealing occurs, the gate has tight dimensional tolerances. When the gate valve is closed, high-pressure fluids may apply loads over 100,000 pounds to a face of the gate. Under these loads, if the face of the gate is not smooth, flat, and parallel to the other face, the gate may be difficult to move, and it may not form a tight seal. High points on the faces can both cause leaks and increase the friction between the gate and seat rings. To prevent the gate from seizing or leaking, its faces are typically ground and lapped to remove these high points. A flat, smooth gate is also less likely to leak under the high pressures.
Generally, these tight tolerances are attained by manufacturing the gate in a particular sequence of steps. Initially, a first face of the gate is coated with a protective material. After coating, the deposited coating material typically is not flat or smooth, so the first face is then ground and lapped by using an opposing, second face, which is uncoated, as a reference, i.e., as the surface from which high points are measured during their removal. One way to do this is by supporting the second face with a flat surface and grinding the coating off the first face. The second face is used as a reference because, before the coating is applied, it is generally flat and, thus, serves as a reliable indicator of high points on the first face. Then, after the first face is coated and ground, the second face is coated, and the grinding process is repeated for this face. When grinding the second face, the ground surface on the first face, which was coated first, is used as a reference. After both sides are ground, they may be lapped to further refine their surfaces.
This sequence of both coating and grinding one side before coating and grinding the other side adds to the cost of the gate. The cost is increased when coating is performed by one vendor, and grinding and lapping are performed by a different vendor in a different location. Thus, to execute the sequence described above, the gate is shipped back and forth from the coating vendor to the grinding and lapping vendor two times, once for each face of the gate. The second trip incurs shipping costs and increases the time it takes to manufacture a gate, both of which tend to make gates more expensive.